1. Field of the Invention
The present invention relates to an electrostatic chuck for clamping and fixing a material to be clamped such as a semiconductor wafer or a glass substrate for an FPD with electrostatic force.
2. Description of Prior Art
Conventionally, a ceramic dielectric material of an electrostatic chuck is provided for the purpose of controlling the electrical characteristics (See Document 1).
However, if the ceramic structure is exposed to a plasma atmosphere, the structure is subject to corrosion, and the surface roughness is deteriorated. Consequently, there are some cases where variations in the contact state between the surface of the electrostatic chuck and the wafer occur over time, or there are other cases where grains are disjoined from a sintered body and the disjoined grain particles cause the wiring of an LSI circuit to be shorted.
In another conventional example, an alumina ceramic material having a particle diameter of 2 μm or less and a relative density of 99.9% in which the plasma-resistance is improved is applied to an electrostatic chuck (See Document 2). However, in this example, even if the plasma-resistance is improved, there is no description of the electrical characteristics, and it is impossible to perform fundamental functions of a Johnsen-Rahbeck electrostatic chuck which enables great clamping force.
Also, an alumina ceramic which contains titanium oxide of 0.1-1 wt % and has volume resistivity of 100-104 Ωcm has been proposed (See Document 3). However, in this case, it is impossible to obtain electrical characteristics for performing functions of an electrostatic chuck.
Also, an electrostatic chuck in which the volume resistivity is reduced by adding titanium oxide of 0.5-2 wt % to an alumina ceramic has been proposed (See Document 4). This document has disclosed that the resistivity is not reduced in a case of less than 0.5 wt %, and too much current flows in a case of 2.0 wt % or more. It has also disclosed that titanium oxide is precipitated in the grain boundary of alumina ceramics. Although an additive of at least 0.5 wt % is required to reduce the volume resistivity, such an amount of the additive is too much for an electrostatic chuck which requires a strict limitation in contamination with respect to a material to be clamped.
Also, there has been proposed an electrostatic chuck in which alumina of 99% or more is contained, the average particle diameter is 1-3 μm, and the volume resistivity is 108-1011 Ωcm in a temperature of 300-500° C. (See Document 5). However, there is no description of properties of an dielectric material required for an electrostatic chuck which is used in another temperature range, for example, a relatively low temperature such as 100° C. or less.    Document 1: Japanese Patent No. 3084869    Document 2: Japanese Patent Application Publication No. 10-279349    Document 3: Japanese Patent Application Publication No. 2004-18296    Document 4: Japanese Pre-grant Publication No. 6-97675    Document 5: Japanese Patent Application Publication No. 11-312729
The object of the present invention is to provide an electrostatic chuck in which the surface can be kept smooth after being exposed to plasma, so as to protect a material to be clamped such as a silicon wafer from being contaminated with particles, and which is excellent in clamping and releasing a material to be clamped.